It has been recognized in the art that the potential fuel efficiency of an internal combustion engine is highest when the engine is operated unthrottled with a compression ratio in the neighborhood of about 12 to 1. However neither the conventional throttled spark ignition engine which requires a lower compression ratio N or the conventional forms of diesel compression ignition engines which require a higher compression ratio are capable of satisfactory operation in the desired range.
The direct injection stratified charge engine of the late injection type can be operated in the desired compression ratio range with an unthrottled air intake and has a further advantage of relatively broad tolerance to fuel type. However, when operating with an unthrottled air intake at part load, the lean mixtures in portions of the combustion chamber distant from the spark cause the flame to be extinguished before the completion of combustion and thus result in relatively high emissions of unburned hydrocarbons. Use of conventional techniques, such as intake throttling and exhaust gas recirculation, to control hydrocarbon and oxides of nitrogen emissions reduces the desired fuel efficiency of the engine.
To overcome these problems, it has been proposed.sup.* to utilize in the combustion chamber of a late direct injection stratified charge engine one or more catalytic grids to initiate combustion of the mixture of injected fuel and air and to promote relatively complete combustion of the fuel without the need for intake throttling. However tests of such an arrangement reportedly met with only limited success due to inadequate performance that was attributed to insufficient charge motion within the combustion chamber. Further work has since been conducted utilizing a modified form of indirect injection diesel engine with reduced compression ratio and a catalytic grid in the precombustion chamber with some improvement in results. FNT .sup.* "The Catalytic Engine" by R. H. Thring, Platinum Metals Review, October, 1980 pgs. 126-133, Johnson Matthey & Co. Limited, Hatton Garden, London, England.